Imaging apparatuses used in mobile equipment need to have both a high resolution and a small size. In many cases, the reduction in size is obstructed by the size and focal length of an imaging optical lens and the size of an imaging device.
In general, since the refractive index of a material varies depending on the wavelength of light, it is difficult to form an image of light from a subject including information of an entire wavelength range on an imaging plane using a single lens. Therefore, in an optical system of usual imaging apparatuses, a plurality of lenses are arranged along an optical axis direction so that images of red wavelength light, green wavelength light and blue wavelength light are formed on the same imaging plane. Accordingly, the optical system is elongated, resulting in a thicker imaging apparatus.
Therefore, as a technique effective for a reduction in size of imaging apparatuses, particularly for a reduction in thickness, a compound-eye imaging apparatus has been proposed in which a plurality of single lenses having a short focal length are arranged substantially in the same plane (see JP 2002-204462 A, for example). A compound-eye color imaging apparatus includes an optical system having a lens for blue wavelength light, a lens for green wavelength light and a lens for red wavelength light that are arranged on the same plane, and imaging devices respectively corresponding to the lenses. Since the wavelength range of light handled by each lens is limited, it becomes possible to form a subject image on the imaging device using the single lenses. Therefore, the thickness of the imaging apparatus can be reduced considerably.
FIG. 15 is a perspective view showing an exemplary compound-eye imaging apparatus. Numeral 900 denotes a lens array including three lenses 901a, 901b and 901c that are formed integrally. The lens 901a is for red wavelength light and forms a subject image in an imaging region 902a. A red wavelength separation filter (color filter) is attached to pixels (photodetector portions) in the imaging region 902a, and the imaging region 902a converts the formed red subject image into image information. Similarly, the lens 901b is for green wavelength light and forms a subject image in an imaging region 902b. A green wavelength separation filter (color filter) is attached to pixels (photodetector portions) in the imaging region 902b, and the imaging region 902b converts the formed green subject image into image information. Further, the lens 901c is for blue wavelength light and forms a subject image in an imaging region 902c. A blue wavelength separation filter (color filter) is attached to pixels (photodetector portions) in the imaging region 902c, and the imaging region 902c converts the formed blue subject image into image information. The pieces of image information outputted from the imaging regions 902a, 902b and 902c are superposed and synthesized, thereby obtaining color image information.
Although the compound-eye imaging apparatus described above can achieve a reduced thickness, it has a problem of a poorer resolution compared with a usual single-eye imaging apparatus. In the single-eye imaging apparatus, an imaging device having a large number of pixels (photodetector portions) arranged in an image forming plane converts incident light into image information. In order to take out color information at each position, the pixels respectively are provided with wavelength separation filters (color filters) in a Bayer arrangement. In other words, the wavelength separation filters transmitting green light are arranged in a checkered pattern so as to correspond to the arrangement of the large number of pixels arranged in a matrix, and the wavelength separation filters transmitting red light and those transmitting blue light are arranged alternately in the rest of the pixels. Each of the pixels only outputs the color information corresponding to the wavelength range of the light that is transmitted by the wavelength separation filter provided in that pixel and does not output color information corresponding to the wavelength range of the light that is not transmitted thereby. However, since the pieces of color information of three colors are known to be correlated in a local region of an image (see Hiroaki KOTERA and two others, “Representation of Full Color Image from a Single Color Image Using Color Correlation” Proceedings of 1988 Annual Conference of The Institute of Image Electronics Engineers of Japan 20, pp. 83-86 (1988), for example), the green color information can be estimated from the red or blue color information. Utilizing such characteristics, the missing color information is interpolated in the imaging device with the wavelength separation filters in the Bayer arrangement. Accordingly, it is possible to obtain a color image having a resolution corresponding to pixels as many as the total number of pixels. For example, in an imaging device with 1,000,000 pixels, 500,000 pixels detect green color information, 250,000 pixels detect blue color information, and 250,000 pixels detect red color information. However, by the above-described interpolation, it is possible to obtain color information having a resolution corresponding to 1,000,000 pixels for each of red, green and blue.
On the other hand, in the compound-eye imaging apparatus, since each of the imaging regions corresponding to the respective colors acquires any of red, green and blue color information, a color image corresponding to pixels as many as the pixels in that imaging region is achieved. For example, in the case where red color information, green color information and blue color information are acquired by three imaging regions each with 250,000 pixels, 750,000 pixels in total are needed, but the resolution of a color image obtained by superposition corresponds to 250,000 pixels.
As a method for improving the image resolution, there is a known technology called “pixel shifting” in which plural pieces of image information whose positional relationships between a subject image and pixels are shifted from each other are acquired by shifting the relative positional relationship between an optical system and an imaging device time-wise or by separating a light beam into plural beams using a prism and making them to enter plural imaging devices and then synthesized so as to achieve a high-resolution image (see JP 10(1998)-304235 A, for example). In this case, the optimal amount of shifting is determined by the direction of shifting and the number of pieces of image information to be acquired. For example, in the case of synthesizing two pieces of image information, when the relative positional relationship between the subject image and the pixels is shifted by an odd multiple of one-half the arrangement pitch of the pixels (in the following, referred to as the “pixel pitch”) between the two pieces of image information, it is possible to obtain the highest resolution image. This technology is applicable as long as it is possible to acquire plural pieces of image information whose relative positional relationships between the subject image formed by the lens and the pixels of the imaging device are shifted from each other, regardless of the method of shifting. In the present invention, the relative positional relationship between the subject image and the pixels of the imaging device that makes it possible to acquire the plural pieces of image information whose relative positional relationships between the subject image and the pixels of the imaging device are shifted from each other and to synthesize these plural pieces of image information so as to obtain a high-resolution image is referred to as a “shifted pixel arrangement.”
In the compound-eye imaging apparatus, a high-resolution image also can be obtained as long as the relative positional relationships between the subject image and the pixels are shifted between plural pieces of image information, in other words, the shifted pixel arrangement can be achieved.
For example, JP 2002-209226 A mentions that, in a compound-eye imaging apparatus using a plurality of lenses to form a plurality of subject images on a plurality of imaging regions, a high-resolution image is obtained by arranging the plurality of lenses and a plurality of imaging devices so that the individual subject images are formed so as to be shifted in a direction connecting optical axes of the lenses, thus achieving the shifted pixel arrangement.
Also, in the compound-eye imaging apparatus, it is possible to determine the distance to a subject using a parallax generated due to the difference between the optical axes of the plurality of lenses.